Lizards are also tormented by mites of their own; they can die from an infestation, and the survivors are likely to have their growth stunted. But when they’re attacked, they go through a different sort of change: they alter their unborn offspring. A lizard infested by mites produces babies that are bigger and faster than those born of healthy parents. A healthy baby lizard will have a growth spurt in its first year and then grow more slowly for the rest of its life. But a lizard born to mite-ridden parents will grow fast for its first two years or more. Lizard mothers apparently can program the growth of their offspring to adapt to the presence of parasites. If there are no mites around, their offspring can grow slowly and live a long life. But if mites turn up, it pays to grow faster in order to reach a healthy weight as an adult, even if that means dying sooner.
And if a host is doomed to die, it can do its best to spare its kin. Worker bumblebees spend their days flying from flower to flower, collecting nectar and bringing it back to their hive. At night they stay in the hive, kept warm by the heat of thousands of flapping wing muscles. On its travels for nectar, a bumblebee may be attacked by a parasitic fly, which lays an egg in its body. The parasite matures within the bumblebee, and in the warmth of a beehive its metabolism runs so quickly that it can finish growing up in only ten days. The fly emerges from its host and can infect the rest of the hive. Yet, many parasitic flies don’t get that luxury because their host does something strange: it starts spending its nights outside the hive. By staying out in the cold, the worker slows down the parasite’s development. It also prolongs its own life. The combined effect makes it unlikely that the parasite will ever make it to maturity before the bee itself dies. In this way, the bumblebee prevents an epidemic from breaking out in its hive.
As cunning as these kinds of counterattacks may be, parasites can evolve counter-counterattacks. If a cow avoids manure to keep away from the lungworms it holds, the parasites will leave the manure. When a lungworm drops to the ground in the manure, it bides its time until light strikes it. That is its signal to climb upward until it reaches the surface of the manure. It begins to hunt around for a species of fungus that is also a parasite of cows—a species that also responds to light by growing little spring-loaded packages of spores. As soon as the lungworm touches the spore package, it latches on and climbs up to the top. The fungus catapults itself six feet into the air and soars away from the manure. The lungworm rides it like a puddlejumper, and out of range of the manure it has better odds of being eaten by a cow.
Study arms races long enough, and you start to imagine that hosts and parasites could carry each other into the clouds, each driving the evolution of its counterpart so hard that they become all-powerful demigods hurling lightning bolts at each other. But of course the race has limits. When Kraaijveld set his wasps against his fruit flies, the fruit flies reached a 60 percent resistance to the wasps after only five generations, but in later generations the resistance simply stayed there at 60 percent. Why didn’t it keep rising to 100 percent, creating a race of perfectly immune flies? Fighting parasites comes at a high cost. It requires energy to make the necessary proteins—energy that can’t be channeled somewhere else. Kraaijveld set his flies selected for wasp-fighting in competition against regular flies for food and found that they fared badly. They grew more slowly than the flies that were still vulnerable to the wasps, they died young more often, and when they grew into adults they were smaller. Evolution doesn’t have an infinite arsenal to offer hosts, and at some point they have to relent, to accept that parasites are a fact of life.
When Darwin set out to write The Origin of Species, figuring out how natural selection works wasn’t his ultimate goal. It was really only a means to an end—to explain the title of his book. After branching and growing for 4 billion years, the tree of life today wears a heavy crown. Scientists have found 1.6 million species, and they may be only a sliver of Earth’s full diversity, which may be many times larger. Darwin wanted to know how that diversity came to be, but he didn’t know enough about biology to find the answer. Now that scientists have a better understanding of heredity and how genes rise and fall over the generations, they’re closing in on how species actually come into being. And they’re finding that the race between hosts and parasites is crucial once again. It may account for much of life’s dense evolutionary canopy.
A new species is born out of isolation. A glacier may cut off a pocket of mice from the rest of their species, and over the course of thousands of years they may develop mutations that make them unlike the rest of the mice and unable to mate with them. A single species of fish may come into a lake and some of its members may start specializing in feeding on the mucky bottom, others in the clear shallows. As they evolve equipment for each kind of life, crossbreeds will turn out badly suited to either one. Natural selection will push them apart, and they will stay more and more with their own until they form separate species.
The life of a parasite encourages new species to form. Parasites can adapt to a single nook in a host—a curl of the intestines, the heart, the brain. A dozen parasites can specialize on the gill of a fish and subdivide it so precisely that there’s no competition between them. Specializing on particular host species makes parasites even more diverse. A coyote will eat just about anything on four legs, and partly as a result, there is only one species of coyote in all of North America. Unlike coyotes and other predators, many parasites are under the Red Queen’s control. A parasite that prefers many different hosts has to try to play the Red Queen game with all of them, like a chess player running frantically between a dozen games he’s playing at once. If another parasite should undergo a mutation that makes it prefer only one host, all of its evolutionary effort will be focused on that host alone. The hosts don’t even have to be an entire species—if just a population of the host is isolated enough, it will pay for the parasite to specialize only on them. With parasites focusing so much on a species or a fraction of a species, they leave room for other parasites to evolve.
As new species are born older ones are going extinct. Species disappear when they are outcompeted, when their numbers shrink down below a critical threshold, or when the world changes too quickly for them to adapt. Lineages of parasites may be able to resist extinction better than those of free-living creatures. While parasites tend to be specialists, they also dabble a little from time to time. Sometimes a new host will turn out to be a good home, and the parasite may found a new species. Tetrabothriid tapeworms are still with us, living in puffins and gray whales, for example, but the pterosaurs and ichthyosaurs in which they lived 70 million years ago are not. The diversity of parasites is like a great lake, with big streams of new species flowing in but only a trickle flowing out into extinction.
Take all these reasons together, and it’s not so surprising that there are so many species of parasites. There are about four thousand species of mammals, and aside from a few rabbits and deer waiting in some obscure forest to be discovered, that number is firm. But there are five thousand species of tapeworms known so far, and new species are discovered every year. There are two hundred thousand species of parasitic wasps. The insects that are parasites of plants number in the hundreds of thousands as well. Add them all up, and the majority of animals are parasitic. Untold thousands of fungi, plants, protozoa, and bacteria also proudly bear the title of parasites.
It’s now becoming clear that parasites may have pushed their hosts to become more diverse as well. Parasites don’t attack an entire species in the same way. The parasites in a particular region can specialize on that population of hosts, adapting to that local set of host genes. The hosts evolve in response—but only the hosts in that region, not the species as a whole. This local struggle has produced some of the fastest cases of evolution ever documented—whether they be yucca moths and the flowers where they lay their eggs, snails and their flukes, or flax and their fungi. And as these populations of hosts fight off their dedicated parasites, they become genetically distinct from the rest of their species.